Add Python 3 Train unit tests

This is a mechanically generated patch to ensure unit testing is in place
for all of the Tested Runtimes for Train.

See the Train python3-updates goal document for details:
https://governance.openstack.org/tc/goals/train/python3-updates.html

Change-Id: I7d0a996b33d4d1eec436f92fbd390968cd37630c
Story: #2005924
Task: #34232

Author: Corey Bryant

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.3
-envlist = py37,py36,py27,pep8
+envlist = py27,py37,pep8
 skipdist = True
 
 [testenv]